Decomposition analysis of a large magnetotelluric dataset from the Nasr-Abad salt diaper, Central Iran

Articles in Press

Document Type : Research Article

Authors

1 Assistant Professor, Institute of Geophysics, University of Tehran, Tehran, Iran

2 M.Sc student, Institute of Geophysics, University of Tehran, Tehran, Iran

3 Ph.D., Iranian Gas and Engineering developmentcompany, Iran

Abstract

We investigated an MT dataset composed of 284 broadband (10-4-3414 sec) MT stations along seven profiles to unravel the electrical properties of sub-surface structures in the Nasr-Abad region, west Central Iran. The region is composed of five Tertiary salt diapirs developed along the Abshirin-Shurab strike-slip fault zone. The MT profiles are extended perpendicular to the general trend of the Zagros orogenic belt (in an SW-NE direction) which is one of the main structural elements controlling regional deformation in the Iranian plateau.
The analysis of impedance data shows that a more complex conductivity structure is expected beneath the SW of the profile. a shallow conductive layer appears throughout the study region which extends to the deeper part beneath the NE of the profile. Furthermore, The Abshirin-Shurab fault significantly influences the apparent resistivity at the SW end of most profiles.
In the next step, we characterize the structural dimensionality of MT data by commonly used Bahr rotational invariants (κ, μ, η, ∑) and the phase tensor skew angle (β). The phase-sensitive skew (η), the regional 1-D indicator (μ), and β skew angle depend on the phase information inherent in the impedance tensor. Therefore, they are affected primarily by large-scale induction anomalies and are immune to low-frequency galvanic distortions. The thresholds assigned for μ, η, and β are 0.1, 0.3, and 3˚, respectively. The skew values calculated from the Nasr-Abad MT data set suggest that the regional conductivity structure is 2D rather than 3D or 1D as the calculated η remains below 0.3 and μ above 0.1. The appropriate category of data is therefore, responses from a regional 2D structure contaminated by galvanic distortion effects.
   We applied the phase tensor analysis for regional strike determination. It does not require any assumption about regional conductivity structure, and its results are not susceptible to galvanic distortion. The method determines the electrical strike from the axis direction of phase tensor ellipses. The analysis reveals a scatter pattern of strikes at short periods (<1 sec) due to the small sampling area of EM fields at these periods. As the period increases an strike azimuth, preferentially aligned N30°W is obtained for the regional geoelectric structure.

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References

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Iranian Journal of Geophysics

Articles in Press, Accepted Manuscript
Available Online from 26 September 2023

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